The present invention relates to an arrangement and process for interpolating a measured signal.
Sensor systems with incremental and absolute measurement and increasingly, for example for angular position and position measurements, the combination of both measurement techniques in high resolution absolute measuring systems are used for industrial purposes. There is a continual requirement for recording absolute position and relative displacement in measuring systems but it is often limited to purely relative measurements since for example the incremental sensor systems suited to this purpose are more cost-effective to produce.
We explain below the basic requirements for high resolution measured signals based on the digitalization of angular position and position measuring systems, whereby our observations may equally be attributed and applied to other physically/chemically derived measured values.
In order to use an incremental measuring system to record absolute position as well, other auxiliary units are required, which determine on the one hand the absolute starting position and permanently calculate on the other hand the incremental signals relative to it sequentially and directionally. Besides the ON/OFF effect of the supply voltage, interference to the mains or faults in transfer of the incremental signals also cause a loss of the absolute signal value in the need to establish a new reference each time. For machines in continuous operation in environments which are prone to interference in particular these measuring systems are unreliable and problematic in an otherwise uninterrupted production process.
The conventional absolute measuring system is again very costly in the implementation of its sensor system and reaches its limits, particularly where high resolutions of the angular and linear distances traveled also in the high speed range are required.
Combined measuring systems have therefore increasingly become established and have absolute angular position/position measuring sensors with resolutions of for example xe2x89xa78 bits, i.e. angular distances which are smaller than 1.5xc2x0 and linear distances of xe2x89xa650 xcexcm.
Useful measured signals totally converted to absolute values in the angular position/position measuring sensorxe2x80x94as described in more detail in the application for Swiss patent CH210599xe2x80x94require in particular a careful method of recording and evaluating the measured signals. Between the absolute values which are determined separately in these systems there are incremental values lying between two absolute values which are used to create the total absolute value. The analog-digital converters with comparator technologies used for interpolating the incremental values have the disadvantage that the cost for resolutions of even 5 to 6 bits are too high and the required hardware integration approaches its limits.
For every step increase in resolution, the cost of hardware integration increases by a factor of 2. This technology is shown in FIG. 1 of the drawing. The measured signal U1 can change e.g. linearly from 0 up to U1max. U1 is applied to a voltage divider Sp, whose pickoffs are all coupled individually to an operational amplifier COMP, to which the reference voltage U2 is also fed. Each of the operational amplifiers COMP switch, whenever the pickup voltage fed to them has reached the preset threshold. The switching of the operational amplifiers determines each particular intermediate value reached, which is then added to the neighboring absolute value.
The above mentioned patent specification CH210599 points to the expected increase in processing speed of semiconductor technology as a practicable solution to overcoming the high cost of higher resolutions and suggests carrying out until that time additional sine/cosine signals and undertaking higher resolution AD conversion externally.
The object of the present invention is to create an interpolation circuitry as well as a process for interpolating analog measured variables which operates with higher speed and accuracy and overcomes the above mentioned disadvantages.
This object is achieved by an arrangement for interpolating an analog measured signal which is dependant on the measured variable, and which is fed to a voltage divider (Sp) whose pickoff signals are fed to comparators, from whose output signals the interpolated measured variable is determined, wherein the pickoff signals are fed by means of switches (S1 to S8) to comparators (Comp, Compxe2x88x921) whose number is smaller than that of the pickoff signals and the switches (S1 to S8) connect the pickoff signals consistently to comparators (Comp, Compxe2x88x921).
It is generally understood that by comparator is meant a suitable threshold switch which operates sufficiently fast and with sufficiently high resolution.
The interpolator or interpolation according to the invention makes particular use of the continuity of functions for measured signal processing used for determining measured variables, e.g. with trigonometric functions for SIN and COS signals. Below we explain the beneficial design of the interpolator or interpolation method using the example of SIN/COS signal processing of position-dependent variables, whereby the arrangement as well as its implementation is basically suitable for and is of benefit for use in other analog and especially continuous analog measured signal flows.
A sensor system must record measured values as continuously as possible if it is to be used also in dynamic applications. This ensures the fastest possible access to measured values at any time since they are already processed when they are called up and may be outputted in xe2x80x9creal timexe2x80x9d. Special measures must be taken with such xe2x80x9creal timexe2x80x9d measuring systems whose clock-pulse rate for logical signal processing is also used for interpolating analog values. Apart from the increasing cost of for example hardware logic to interpolate higher resolutions, it is particularly important to take into account the ever smaller signal amplitudes for each step in resolution critical for accuracy and sensitivity of the recording process in addition to time-critical switching procedures.
The object of the invention is also achieved by a method for interpolating an analog measured signal which is dependant on the measured variable, and which is fed to a voltage divider (Sp) whose pickoff signals are fed to comparators, from whose output signals the interpolated measured variable is determined, wherein the pickoff signals are fed by means of switches (S1 to S8) to comparators (Comp, Compxe2x88x921) whose number is smaller than that of the pickoff signals and the switches (S1 to S8) connect the pickoff signals consistently to comparators (Comp, Compxe2x88x921).
The method of interpolation described in the invention solves the above-mentioned object by selecting a smaller number of comparators to use for comparison than the number of reference points given by the resolution and by using switches to feed these consistently to at least one comparator for signal evaluation.
This beneficial method of interpolation has limits in the resolution of the analog values given by the accuracy of the resistor dividers to be formed and the offset values of the comparators. Since the resistance values are in the thousandths range of the accuracies which can be achieved by semiconductors and the offset values of the comparators are also in the thousandths range of the useful signal, resolutions of approx. 8-bit can still be achieved at the first stage of interpolation without a great deal of extra expenditure. The advantage of the reduced number of comparators can find an additional use, however, in that an on chip offset adjustment is carried out which then allows the range of resolution to be expanded up to 10-bit with correspondingly more expensive resistorsxe2x80x94e.g. parallel switching of the resistors allows a corresponding increase in accuracy.
In many applications, e.g. servo drives for position control, there is a requirement for the highest possible total resolutions, which are in the range of 22 . . . 24 and even up to 26 bits in rotary encoders. The same is true of stiff position controls of linear drives with distance resolutions of a few nanometers. In this case, it depends above all on the precision recording of the relative movement of the adjustment device to be measured, in order to derive the speed or acceleration for control from the value of the measured angle or position. Thus for angular position and position encoders the highest possible accuracies up to total-basic resolutions of e.g. 17 to 18 bits can be produced and beyond that only relatively evaluatable angular position/position resolutions of up to 22 to 26 bits.
In patent specification CH210599 for example the total absolute value of 17 to 18 bits is made up of a 12-bit basic absolute value and a 5 to 6-bit incremental value (also called the fine value) gained by means of interpolation. Here, even an interpolation of 8 bits would result in a total absolute value of 20 bits. And using the appropriate resistor dividers as well as offset compensation measures the total resolution can even be increased up to 22 bits.
In a further embodiment of the invention the resolution of the interpolation can be increased still further, up to 12 bit and beyond so that total absolute values of 24 to 26 bits can be achieved. This is accomplished by imposing a second interpolation stage on top of the first interpolation stage in which the individual difference values of the calculated first stage are amplified and further interpolated in the second stage. The measured values gained from the first interpolation stage are described as fine values and those from the second interpolation stage as finest values. The total absolute value of e.g. 24 bits being formed in this way comprises the basic absolute value of e.g. 12 bits and the interpolated value of e.g. 12 bits which is again formed from a for example 8-bit fine value and 4-bit finest value.
The interpolation built up according to the invention allows the recording of a total absolute value to be determined under real time conditions. The total absolute value may be formed outside a sensor by for example feeding the basic absolute value and the SIN/COS signals to the evaluation electronics with the inventive interpolation and combining them here synchronously. It is particularly advantageous to create the interpolation in the sensor itself since this allows highest accuracy and resolution in creating the total absolute value in interference sensitive ambient conditions with maximum processing speeds. The very high resolutions also require appropriate treatment of the calculated interpolation values.
According to a further embodiment of the invention an adaptive interpolation filter is proposed in which the difference between the filtered measured value and the current measured value is fed in to a clocked evaluation unit. The differences are added together so that, when certain limit values are reached, the last filtered measured value is subsequently increased or reduced consistently by one resolution step and brought to the measured value output.
In a further embodiment of the invention a digital filter is formed so that the last determined direction of movement of the supplied measured values is used for various evaluation of the current measured values by at least one resolution step in the direction of the deviation from the measured value output. It is beneficial to continue the evaluation according to the deviation of the current measured values in comparison to the values used for the measured value output as well as by time according to inquiry clock-pulse cycles. Adaptive digital filtering has an advantage here in that the deviations to be determined relate solely to measured value deviations themselves and may therefore be used in the total measuring range without the influence of filter timesxe2x80x94from standstill up to the maximum speed of the changes in measured values. Diverse evaluation by filter values according to direction of movement is determined by the bearing (play), elasticity as well as frictional conditions present predominantly in the mechanics. As a rule, therefore, the interference values are in total lower in the direction of movement so that filter measures are weaker than in the opposite direction and therefore can be carried out beneficially in many cases asymmetrically. But there are also other interference values such as e.g. control characteristics of the control unit or asymmetric signal interference values which can be optimally adjusted by the filter properties according to the direction of movement and possibly be carried out with opposite evaluation if required and under special circumstances. Optimizing asymmetric filtering of measured signals offers the benefit of achieving the fastest possible and interference free measured signal processing in application.
It is often the case in industrial applications however, especially at low speeds or standstill, that high measured signal resolutions are required, so that interpolation and also filtering can be chosen or automatically switched to suit the speed of the adjustment device to be measured. This allows interpolation to be chosen for example at standstill at the highest resolution or at the highest possible resolution proportional to a particular speed, so that the measured value can still be recorded in the resolution step under real time conditionsxe2x80x94i.e. during one clock pulse cycle of measured value processing e.g. at 30 to 50 MHz. By including the speed of the measurement device it is possible to achieve e.g. the greatest interpolation step including the filtering optimized to it during measured signal processing.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an arrangement and process for interpolating a measured signal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.